The reflectivity of an electrochromic mirror device can be varied by applying a voltage over the device. This makes them ideal for use in rear view mirrors of vehicles. They solve the danger problem that the vehicle driver gets temporarily blinded after having looked into the reflected headlights of the vehicles trailing behind him. Already many different systems exist but the constant behind these electrochromic devices is that there occurs a change in transparency in a material layer due to the reduction or oxidation of an ionic species that is displaced under the action of an electric field. The ionic species can be a proton, or a lithium ion or—in rare cases—sodium or potassium ions.
The existing systems differ in the kind of electrochromic layers that are used. The electrochromic layer is the layer that changes transparency when the ionic species are extracted or incorporated into the layer. Electrochromic materials exist that e.g. become darker when the ionic species gets into the layer. Tungsten oxide for example will darken when lithium ions are incorporated and reduced into its lattice (colouring on reduction or cathodically colouring). Nickel oxide will darken when lithium atoms are expelled out of the network as ions (colouring on oxidation or anodically colouring). Although the electrochromic material is generally a solid it can be incorporated in a polymeric matrix. By combining a cathodically with an anodically colouring layer the absorptive effect of both is added. Both layers are preferably separated by an ion conductive but electron resistive layer, usually referred to as the ion conductor.
This ion conductor is another variant in the design of the electrochromic mirror: it can be liquid or solid, or it can be a liquid held in a solid or gel matrix. It can be based on inorganic materials as well as on organic materials.
For the purpose of this application, only all solid-state electrochromic mirror devices are considered. The all solid state layers of such device can conveniently be deposited in vacuum coating installations, one layer after the other. All solid state layers are most stable compared to their gel or liquid counterparts. The layers can be made very thin and hence can be made very light. One can therefore make such electrochromic stacks on a light flexible substrate such as a reflective metal substrate and attach them to a convenient carrier such as a—possibly curved—glass pane. Such an assembly is much lighter than the known assemblies wherein the layers are held between at least two glass panes, considerable adding to the weight of the mirror. As a consequence, the higher weight results in a lower frequency of vibration of mirrors that causes a blurred rearview.
However, because the thicknesses of the layers in the electrochromic stack come close to the visible wavelength, the reflection at the different interfaces leads to interference of the light rays. This results in a distorted colour of the reflected image that moreover depends on the angle under which the rays are reflected (sometimes called iridescence). Such a situation is very prominent when the mirror is in the bleached state. A too much distorted colour will lead to confusion for the driver and is not acceptable (see e.g. EU Directive 2003/97/EC, section 3.5 of Annex 2). This problem does not occur for systems that use polymer or liquid electrochromic layers as there the thickness of at least one of the layers of the device covers several wavelengths which leads to a loss in coherence and hence no interference is observed. Typically the thickness of this kind of systems is 100 to 150 micrometer (see e.g. US 2003/0112489).
U.S. Pat. No. 5,673,150 (equivalent to EP 0652462) describes for example such an all solid state electrochromic stack for use in an anti-glare mirror. The stack comprises an indium tin oxide (ITO) transparent conductive layer, an iridium oxide layer (IrOx) as an anodically colouring electrochromic layer, tantalum oxide (Ta2O5) as an ion conductor and tungsten oxide (WO3) as a cathodically colouring electrochromic layer. The ionic species is the hydrogen ion (H+). The disclosure addresses the problem of the interference at the interfaces at the layers and solves this by introducing a tin oxide layer (SnO2) between the iridium oxide and the tantalum oxide layer. Although the problem seems to be alleviated, it appears not to be completely gone as the reflectorgrams in the disclosure still shows prominent peaks and valleys.
U.S. Pat. No. 6,768,574 likewise discloses an all solid state rear view mirror for use in vehicles comprising all solid state layers of the type mentioned above but wherein at least one of the layers is ‘nanostructured’, the term being defined as: (a) having a structure with a crystalline order of nanometric range (domains between 2 to 20 nm), (b) having crystalline domains of nanometric dimensions (1 to 20 nm) embedded in an amorphous matrix of the same or a different compound or, (c) having a structure formed by multiple layers of nanometric thickness (2 to 100 nm). The embodiments show alternative materials with vanadium oxide (V2O5) in stead of iridium oxide and silicon carbide (SiC) as ion conductive layer in stead of tantalum oxide. In one of the embodiments the ion conductor layer (SiC) is an amorphous layer wherein crystallite SiC nanometric (2 to 3 nm in diameter) spherical particles are embedded. The nanostructuring is claimed to lead to a reduced electronic conductivity and a better ion retention, leading to higher switching speeds. No mention is made of an interference problem between the different layers.
WO 94/15247 describes a transparent electrochromic device wherein optical tuning layers are provided to enhance the transmission of the device and thereby reduce the iridescence when in the bleached state.
Japanese abstract JP 2005 330148 describes an anti-fogging element comprising a stack of layers with specified optical thicknesses to one another and a porous hydrophilic layer on top to prevent that fog would deteriorate the vision of the—possibly electrochromic—mirror.